System and method for characterizing tissue based upon split spectrum analysis of backscattered ultrasound

ABSTRACT

A system and method are disclosed that facilitate characterizing vascular plaque tissue based upon spectral analysis of intravascular ultrasound echo signal segments. In particular, split spectrum analysis of an integrated backscatter parameter introduces a spectral resolution component to parameterization of received intravascular ultrasound echo signal segments. The resulting parameter values for each of multiple bands within a larger frequency band supported by an ultrasound system are applied to plaque tissue characterization criteria to render a plaque tissue characterization corresponding to the corresponding ultrasound echo signal segments.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.11/767,807, filed on Jun. 25, 2007, now U.S. Pat. No. 8,162,836, whichclaims priority of Waters U.S. provisional application Ser. No.60/816,286 filed on Jun. 23, 2006, entitled “Split Spectrum Analysis forIntravascular Ultrasound Tissue Classification,” the contents of whichare expressly incorporated herein by reference in their entiretyincluding the contents and teachings of any references containedtherein.

FIELD OF THE INVENTION

The present invention generally relates to the field of imaging systems,and more particularly to intravascular ultrasound imaging systems andmethods for diagnosing vascular disease.

BACKGROUND

The development of new medical technologies has provided an increasingnumber of options available to doctors for the diagnosis and treatmentof cardiovascular diseases. The availability of such equipment hasimproved the ability of doctors and surgeons to detect and treatcardiovascular disease. Intravascular imaging technologies have enableddoctors to create and view a variety of images generated by a sensorinserted within a vasculature. Such images complement traditionalradiological imaging techniques such as angiography by providing imagesof the tissue within vessel walls rather than showing a two dimensionallumen image.

Intravascular ultrasound (IVUS) analysis finds particular application toa system and method for quantitative component identification within avascular object including characterization of tissue. It should beappreciated that while the exemplary embodiment is described in terms ofan ultrasonic device, or more particularly the use of IVUS data (or atransformation thereof) to characterize a vascular object, the presentinvention is not so limited. Thus, for example, using backscattered data(or a transformation thereof) based on ultrasound waves or evenelectromagnetic radiation (e.g., light waves in non-visible ranges) tocharacterize any tissue type or composition is within the spirit andscope of the present invention.

Imaging portions of a patient's body provides a useful tool in variousareas of medical practice for determining the best type and course oftreatment. Imaging of the coronary vessels of a patient by techniquesinvolving insertion of a catheter-mounted probe (e.g., an ultrasoundtransducer array) can provide physicians with valuable information. Forexample, the image data indicates the extent of a stenosis in a patient,reveals progression of disease, and helps determine whether proceduressuch as angioplasty or atherectomy are indicated or whether moreinvasive procedures are warranted.

In an ultrasound imaging system, an ultrasonic transducer probe isattached to a distal end of a catheter that is carefully maneuveredthrough a patient's body to a point of interest such as within acoronary artery. The transducer probe in known systems comprises asingle piezoelectric crystal element that is mechanically scanned orrotated back and forth to cover a sector over a selected angular range.Acoustic signals are transmitted and echoes (or backscatter) from theseacoustic signals are received. The backscatter data is used to identifythe type or density of a scanned tissue. As the probe is swept throughthe sector, many acoustic lines (emanating from the probe) are processedto build up a sector-shaped cross-section image of tissue within thepatient. After the data is collected, an image of the blood vessel(i.e., an IVUS image) is reconstructed using well-known techniques. Thisimage is then visually analyzed by a cardiologist to assess the vesselcomponents and plaque content. Other known systems acquire ultrasoundecho data using a probe comprising an array of transducer elements.

In a particular application of IVUS imaging, ultrasound data is used tocharacterize tissue within a vasculature and produce images graphicallydepicting the content of the tissue making up imaged portions of avessel. Examples of such imaging techniques for performing spectralanalysis on ultrasound echoes to render a color-coded tissue map arepresented in Nair et al. U.S. Pat. No. 7,074,188 entitled “System andMethod of Characterizing Vascular Tissue” and Vince et al. U.S. Pat. No.6,200,268 entitled “Vascular Plaque Characterization”, the contents ofwhich are incorporated herein by reference in their entirety, includingany references contained therein. Such systems analyze responsecharacteristics of ultrasound backscatter (reflected sound wave) data toidentify a variety of tissue types found in partially occluded vesselsincluding: fibrous tissue (FT), fibro-fatty (FF), necrotic core (NC),and dense calcium (DC).

When characterizing the response of tissue that has been subjected toultrasound waves, parameter values are considered at a data point in animaged field. Based upon response characteristics (e.g., power spectra)of known tissue types, tissue at the data point is assigned to aparticular tissue type (e.g. necrotic core). Known systems utilize anintegrated backscatter parameter that represents a power response over afrequency band. The integrated backscatter parameter generallyrepresents a measure of total reflected ultrasound power at a particularpoint within a vasculature over a specified frequency band.

SUMMARY OF THE INVENTION

In accordance with the present invention a method and a supportingsystem operating according to computer-executable instructionscharacterize tissue components of plaque within blood vessels. For agiven ultrasound echo signal segment, an integrated ultrasoundbackscatter parameter value is determined for each of at least twodiffering frequency ranges (bands). The at least two integratedbackscatter parameter values are thereafter processed in combination tocharacterize a volume of tissue to which the ultrasound echo signalsegment corresponds.

More particularly, a method for characterizing plaque tissue inaccordance with received intravascular ultrasound echo segments isdescribed herein. The method includes collecting analog ultrasound echosignals from a region of interest in a portion of an imaged body. Suchregion of interest is, for example, a blood vessel. Thereafter, thesystem computes, from RF-domain ultrasound echo signal segment dataderived from the analog ultrasound echo signals, spectrally resolvedinformation over an operating frequency range. The spectrally resolvedinformation includes: a power spectrum rendered by performing spectralanalysis, and a system transfer function.

Thereafter, the system determines, in each of at least a first sub-bandand a second sub-band within the operating frequency range, anintegrated backscatter parameter from the power spectrum and systemtransfer function. The frequency ranges are, by way of example adjacent,but not overlapping. However, in other embodiments the first and secondsub-bands have overlapping ranges or band gaps between them.

The system thereafter compares integrated backscatter values from atleast the first sub-band and the second sub-band to obtain at least oneintegrated backscatter comparison parameter value. The comparisonparameter value is for example, is based upon signed difference value.Alternatively, the comparison value is based upon a ratio of the firstand second sub-band integrated backscatter values.

Thereafter, the system applies the at least one integrated backscattercomparison parameter value to at least one tissue characterizationcriterion for rendering a plaque tissue characterization.

BRIEF DESCRIPTION OF THE DRAWINGS

While the claims set forth the features of the present invention withparticularity, the invention, together with its objects and advantages,may be best understood from the following detailed description taken inconjunction with the accompanying drawing of which:

FIG. 1 illustrates a tissue-characterization system suitable forcarrying out a tissue/plaque characterization scheme including anintegrated backscatter parameter generator that generates, for a givenultrasound echo segment, at least a first and second integratedbackscatter parameter value over differing sub-bands of a largerfrequency band;

FIG. 2 illustrates an exemplary graphical depiction of a receivedultrasound echo signal's amplitude over a period of time;

FIG. 3 is an exemplary grayscale ultrasound image of a vessel'scross-section;

FIG. 4 is an exemplary graphical depiction of an apparent backscattertransfer function (ABSTF) for an echo signal segment;

FIG. 5 is an exemplary graphical depiction of an ABSTF for an echosignal segment that has been split into low and high frequency bands forcalculating low and high IB parameter values;

FIGS. 6 a and 6 b each describes a set of processing stages forrendering parametric information for characterizing plaque tissue; and

FIG. 7 is a flowchart summarizing an exemplary set of steps forgenerating integrated backscatter information to generate plaque tissuecharacterization data in accordance with an illustrative embodiment ofthe present invention.

DETAILED DESCRIPTION OF THE DRAWINGS

Characterizing various types of tissue that make up plaque found withinblood vessels facilitates classifying the larger plaque structures inorder to diagnose and treat vascular disease. In the context of thedisclosed system, a parameterized property of interest is a backscatterpower coefficient that is indicative of how strongly a microscopicvolume of tissue scatters ultrasound back to an ultrasound source. Inaccordance with exemplary embodiments system-dependent spectralcharacteristics of the IVUS system 100 are deconvolved from thebackscatter power from tissue using a blind deconvolution operation.Deconvolution is an engineering discipline that improves the fidelity inelectronic signals by removing (i.e., deconvolving) features of thesignal that depend only on the imaging or measurement system (e.g., IVUSsystem). Blind deconvolution is a process in which the system-dependentcharacteristics are not a priori known. Such a process is described, byway of example, in Jirik and Taxt, IEEE Ultrasonics, Ferroelectrics, andFrequency Control, Vol. 53, No. 8, 2006, pp. 1440-1448. The resultingnormalized backscatter power is referred to as the apparent backscattertransfer function (ABSTF). FIG. 4 graphically depicts an example of anABSTF (in logarithmic dB scale) derived from the backscatter powercoefficient for an ultrasound echo signal segment. The ABSTFapproximates the backscatter power coefficient. The ABSTF is depictedover a measured frequency band corresponding to, for example, theoperating frequency spectrum of a system. The area under the ABSTF curve(representative of an echo signal segment's total backscatter power overa given frequency range) normalized (e.g., divided) by the bandwidth(f_(high)-f_(low)) to account for unequal and/or variable bandwidths, isreferred to herein as the integrated backscatter (IB). The IB thusrepresents the average of the ABSTF curve over a specified bandwidth.

The polarity of the signal value is maintained during the IBcalculation. Thus, the portions of the graph above the “0” amplitudelevel indicate a positive contribution to the IB value. The portions ofthe graph below the “0” amplitude level indicate a negative contributionto the IB for a backscatter signal. The IB provides a measure of thefrequency-averaged backscatter power of a particular tissue volume overa designated frequency band from f_(low) to f_(high).

Turning to FIG. 5, the disclosed system and method for characterizingplaque tissue in blood vessels improve the spectral resolution ofIB-based characterization schemes by splitting the operating frequencyrange of an IVUS system into at least two bands. In the exemplaryembodiment the operating range (f_(low) to f_(high)) is split into a lowband (f_(low) to f_(mid)) and a high band (f_(mid) to f_(high)).Alternatively the operating frequency range is split into even moresub-bands. Furthermore, while the illustrative example designates two,non-overlapping, adjacent bands, alternative embodiments utilizeoverlapping bands as well as ones with gaps between one or more bands ofinterest.

Based upon the split frequency range, an IB_(low) parameter value and anIB_(high) parameter value are calculated from the ABSTF in therespective low and high sub-bands of the full operating band. Yet otherexamples of potentially useful IB-based parameters providing enhancedspectral resolution for characterizing plaque tissue (e.g., by applyingto a tissue characterization decision tree) include: (1) a differencebetween the IB_(low) and IB_(high) values, and (2) a ratio of IB_(low)to IB_(high).

Furthermore, other embodiments of the present invention utilize thesplit spectrum of the ABSTF to render other spectrally resolvedparameterized data. Examples include slope and intercept parameters froma linear regression of the ABSTF within each sub-band.

The spectrally resolved parametric data values from at least twodistinct bands within a larger operating frequency range of anultrasound system are thereafter applied to characterization criteria torender a characterization for the tissue corresponding to theintravascular ultrasound signal segment. Experimentation has confirmedthat splitting a larger available frequency band into two or moresub-bands for purposes of rendering a set of IB parameters from a sameintravascular ultrasound echo segment, facilitates identifyingparticular types of tissue (e.g., fibro-fatty and necrotic core) withinplaque deposits in blood vessels.

It is furthermore noted that both analog and/or digital processingmethods are potentially used to define the sub-bands in a split-spectrumanalysis that renders parameter values representing the spectral powerresponse of intravascular ultrasound echo signal segments in specifiedsub-bands (to render the IB parameter values for each spectral band).Furthermore, the various system implementations utilize a variety ofcomponents to analyze and render IB parameters, including: analogcircuitry and digital circuitry as well as hardware/firmware/softwareand combinations thereof.

An exemplary IVUS (intravascular ultrasound) system includes anultrasonic probe device mounted upon a flexible elongate member forinsertion into vasculature. The system furthermore includes a computingdevice comprising memory for storing computer executable instructionsassociated with rendering a set of IB parameter values for a givenultrasound echo signal segment corresponding to particular sub-bands ofa larger band associated with an ultrasound system. Additional computerexecutable instructions stored on the computing device apply the set ofIB parameter values to plaque tissue characterization criteria. In thedetailed description of the exemplary embodiment that follows, likeelement numerals are used to describe like elements illustrated in oneor more figures.

Turning to FIG. 1, a vascular plaque tissue characterization system 100is schematically depicted. An intravascular ultrasound console 110 iscommunicatively coupled to an IVUS catheter 120. The IVUS catheter 120comprises a distally mounted ultrasound transducer probe 122 thatacquires backscatter data (e.g., IVUS data) from a blood vessel. Inaccordance with known IVUS catheters, the catheter 120 is maneuveredthrough a patient's body (e.g., via a femoral artery) to a point ofinterest. The transducer probe 122 is then controlled, via the console110 to emit ultrasound pulses and thereafter receive echoes orbackscattered signals reflected from vascular tissue/plaque and blood.Because different types and densities of tissue absorb and reflect(backscatter) the ultrasound pulse differently, the reflected ultrasoundecho data (i.e., IVUS data) signals transmitted back to the console 110by the IVUS catheter 120, are converted by characterization softwareinto images of vascular objects. It should be appreciated that the IVUSconsole 110 depicted herein is not limited to any particular type ofIVUS console, and includes all ultrasonic devices known to those skilledin the art (e.g., In-Vision Gold and s5™ systems of VolcanoCorporation). It should further be appreciated that the IVUS catheter120 depicted herein is not limited to any particular type of catheter,and includes all ultrasonic catheters known to those skilled in the art.Thus, for example, a catheter having a single transducer (e.g., adaptedfor rotation) or an array of transducers (e.g., circumferentiallypositioned around the catheter) is within the spirit and scope of thepresent invention.

Known imaging applications executed on an IVUS console (e.g. console110) or a communicatively coupled computing device (e.g., computingdevice 130), render a variety of image types from received echoinformation. A first type of imaging application converts ultrasoundecho signal data into gray scale images reflecting the relative strengthof the echo signal returned by the objects within the transducer probe120's field of view. In such imaging applications, the relatively lightand dark regions indicate different tissue types and/or densities.

Other imaging applications, such as a tissue characterizationapplication 132 executed on the computing device 130 communicativelycoupled to console 110, renders tissue type information based upon thespectral (e.g., frequency and power) characteristics of the echoinformation received by the console 110 from the catheter 120. Inaccordance with an illustrative embodiment, IB parameter values arerendered from spectral analysis of the ultrasound echo information. Inparticular, IB parameter values are rendered for each of multiplesub-bands of an operating frequency band of the system 100 bycorresponding IB processors (e.g., Low Band IB Processor and High BandIB Processor). The Low and High Band IB processors are identifiedseparately in FIG. 1. However, in an exemplary embodiment, a singledynamically configurable IB processor renders IB parameter values on thefly from one of multiple selectable frequency sub-bands.

Furthermore ultrasound power spectra from which IB parameter values arecalculated in any of many potential ways. In illustrative examples,described further herein below, the ultrasound power spectrum (dividableinto at least two sub-bands corresponding to Low and High Band IBparameters) is calculated by applying either Fourier or autoregressivemodeling techniques to an ultrasound echo signal segment (see, e.g.,FIG. 2). With reference to FIG. 2, in an exemplary embodiment abracketed segment of a portion of an ultrasound echo signal is digitizedand provided to both the Low Band IB Processor (associated with arelatively low sub-band of an operating frequency range of system 100)and the High Band IB Processor (associated with a relatively highsub-band of the operation frequency range of system 100) of thecharacterization application 132 to render corresponding Low and HighBand IB parameter values for the echo signal segment. Turning briefly toFIG. 3 showing an exemplary gray-scale IVUS image, in an exemplaryembodiment, the system 100 correlates a time of receipt of the bracketedecho signal segment with a range (distance from a transducer element)along a scan line (the dotted horizontal line) in a field of view of anultrasound probe.

The spectral resolution-enhanced IB parameter value sets extracted fromthe echo information rendered by the catheter 120 for a same echo signalsegment are evaluated and applied to tissue characterization criteriathat incorporate the frequency response signatures associated withparticular types of plaque tissue. The IB parameter values arepotentially applied in association with other extracted parametersrendered by other signal processors, to the characterization criteriaincorporated in the characterization application 132, to render a tissuecharacterization for a point within a field of view of the ultrasoundprobe corresponding to an echo signal segment from which the IBparameter values were derived.

It is noted that while a tissue characterization is based on a singleecho signal segment in the illustrative example described above, inalternative embodiments multiple echo signal segments and/or IBparameter values are combined with temporal/spatial neighbors to rendervalues that exhibit improved signal/noise ratios. Thus, multiple echosignal segments from adjacent scan lines or repeated firings on a samescan line can be combined and presented to the Low and High Band IBprocessors. Also, multiple TB parameter values corresponding to thesignal segments are potentially combined. In either case, thecombination potentially improves the overall signal/noise ratio.

A data storage 134 stores the tissue characterizations rendered by thecharacterization application 132 based upon parametric informationgenerated from the Low and High Band IB processors (and potentiallyother extracted parametric data) to the characterization criteria. Thedata storage 134 is, by way of example, any of a variety of data storagedevices, including RAM, cache memory, flash memory, magnetic disks,optical disks, removable disks, SCSI disks, IDE hard drives, tapedrives, optically encoded information discs (e.g., DVD) and all othertypes of data storage devices (and combinations thereof, such as RAIDdevices) generally known to those skilled in the art.

In the illustrative example, the tissue characterization application 132exists as a single application comprising multiple integratedbackscatter parameter value processor components corresponding tomultiple frequency sub-bands for which IB parameter values arecalculated for each ultrasound echo segment. However, in otherembodiments, the characterization application 132 comprises multipleapplications/components executed on one or more computing devices(including multiple processor systems as well as groups of networkedcomputers). Thus, the number and location of the components depicted inFIG. 1 are not intended to limit the present invention, and are merelyprovided to illustrate the environment in which an exemplary systemoperates. Thus, for example, a computing device having a plurality ofdata storage devices and/or a remotely located characterizationapplication (either in part or in whole) is within the spirit and scopeof the present invention.

FIGS. 6 a and 6 b each describes an exemplary set of processing stagesfor rendering parametric information for characterizing plaque tissue inaccordance with different raw IVUS signal processing architectures.Referring to FIG. 6 a, a set of steps are summarized for processing datathat is initially provided in the data domain (i.e., as basebanddata)—requiring significantly less signal data to be stored since thedigitized raw RF signal is converted to data components rather thanpreserving a digitized version of the time domain RF signal. In theexemplary embodiment, wherein the split-spectrum analysis is performedin the RF domain, “remodulation” of the data signal (back to the RFdomain) is performed in preparation for generation of the ABSTF for theselected ultrasound echo signal segments.

During step 600 an ultrasound probe transmits ultrasound pulses andreceives ultrasound echo signals from backscatters within the field ofview defined by a set of scan lines. The received analog echo signalsare passed to a patient interface module wherein, during step 602, theanalog signals are time-gain compensated via amplifier circuitry toaccommodate the inherent drop off of reflected energy as a function ofdistance from the ultrasound probe. Next, during step 604 thecompensated analog signal is filtered and amplified in preparation forextracting a data signal.

Thereafter, during step 606 data is extracted from the compensatedanalog echo signal by a filter and gain stage. The resulting analog datasignal is thereafter digitized during step 608. In an exemplaryembodiment a 16 bit signed digital value is stored for each sample.

In an exemplary embodiment, an averaging operation is performed on thedigitized data during step 610 to improve a signal to noise ratio of thedigitized data. Next, during step 612 a focusing operation is performedto render additional echo image data. Focusing (i.e., beamforming)combines signals from a neighborhood of array elements in order toimprove sensitivity. A similar process can be performed with consecutivetransmissions when using an ultrasonic probe that is comprised of asingle element. Thus, while a signal segment is referred to herein insingular form, the ultrasound echo signal segment referred to herein ispotentially a sum of multiple spatial or temporal echo signals receivedby the IVUS system. It is furthermore noted that for a given region ofinterest, multiple ABSTF functions/curves associated with points on ascan line and neighboring scan lines are combined to render an ABSTF fora given region of interest.

During step 614 additional time-gain compensation is applied to thefocused digitized data signal. Thereafter, during step 616, the datasignal is remodulated. Remodulation transforms data-domain data back toRF-domain data which is utilized by the Low and High Band IB processors.

During step 618 additional compensation operations are performed on thetime-gain compensated time-domain signal.

During step 620 additional operations are performed on the modulateddata to compensate for diffraction effects on the ultrasound echosignal.

Thereafter an echo signal segment corresponding to a region of interestin the ultrasound probe's field of view is selected in step 622.Selection of an echo signal segment, in an exemplary embodiment,comprises selecting an echo signal segment for multiple scan lines andpoints within a scan line to improve signal/noise characteristics. In anexemplary embodiment, the segments are independently processed throughABSTF and IB parameter calculations. The IB parameter calculations foreach echo signal segment are averaged prior to application ofcharacterization criteria during step 630 described herein below.

The power spectrum of the region of interest of the compensatedtime-domain remodulated signal is calculated in parallel with a blinddeconvolution operation to calculate the system transfer function forthe region of interest during step 624. The ABSTF curve (See FIG. 4) isthen calculated in step 626.

Based upon the selection of the region of interest during step 622,during step 628 spectrally distinct parameter values (e.g., IB values)are calculated for each of at least two sub-bands of a device'soperation frequency band in accordance with the above-described IBparameter generation operation. Thereafter, the spectrally resolved IBparameter values for each echo signal segment for potentially multipleecho signal segments processed for a region of interest are submitted totissue characterization criteria for classifying the tissue during step630.

During step 630, the spectrally resolved IB parameters are applied in avariety of ways to tissue characterization criteria. In an exemplaryembodiment, the Low Band IB parameter value corresponds to the IBparameter value from the low end of the system bandwidth (e.g., −20 dBlevel) to the mid-band, the High Band IB parameter value corresponds tothe IB parameter value from the mid-band to the high end of the systembandwidth, and at least one IB parameter value comparison provided tothe characterization criteria includes a difference between the HighBand IB parameter value and the Low Band IB parameter value. Thedifference is a signed value representing the relative magnitudes of theHigh and Low IB parameter values. In yet another embodiment, the atleast one IB parameter value comparison provided to the characterizationcriteria includes a ratio based upon the High Band IB parameter valueand the Low Band IB parameter value.

As noted above, the IB parameter values for multiple echo signalsegments associated with a region of interest are averaged spatiallyand/or temporally during step 626 or 628 to improve signal/noisecharacteristics. The spectrally resolved IB parameter values for each ofthe multiple echo signal segments are combined (e.g., averaged) prior toapplying the averaged spectrally resolved signals to the characterizingcriteria during step 630 to characterize the tissue in the currentregion of interest.

Next, at step 632 if additional regions of interest are available, thencontrol returns to step 622. Otherwise, control passes to step 634wherein the system applies previously determined characterizations forregions of interest to render displayable image data.

Briefly referring to FIG. 6B, the steps (with a B appended to referencenumbers previously described with reference to FIG. 6A) in a timedomain-based system are similar to the correspondingly identified stepsin FIG. 6A. However, the time domain system that operates upon theanalog signal does not perform the Demodulation and Filtering step 606and the Remodulation step 616 that are needed in the baseband-basedsystem represented in FIG. 6A.

FIG. 7 is a flowchart summarizing an exemplary set of steps forgenerating integrated backscatter information to generate plaque tissuecharacterization data in accordance with an illustrative embodiment ofthe present invention. During step 700 the tissue characterizationapplication acquires raw IVUS image data. Next, at step 702 a vascularobject is identified by a user. Thereafter, at step 704 the user selectsa region of interest within an imaging probes' present field of view.

At step 706, the system independently computes in parallel the powerspectrum for an echo signal segment and the system calibrationinformation for the selected region of interest. Such calibrationinformation includes, but is not limited to the point spread functionand system transfer function (e.g., blind deconvolution).

Thereafter, at steps 708 and 710 the system calculates the ABSTF (708)and IB parameters at multiple sub-bands (710) for the selected region ofinterest. During step 712, in an exemplary embodiment a low and highsub-band IB are compared to render a signed difference signal (asopposed to an unsigned magnitude value) that potentially aidscorrelating a signal's parameter value with a particular tissuecharacterization.

Next, during step 714 the values computed during steps 710 and 712 areapplied to characterization criteria to render a tissuecharacterization. If additional regions of interest exist, then controlpasses from step 716 back to step 704. Otherwise, at step 718 thedesignated characterizations for the various regions of interest arecombined into a single displayed image according to the assigned tissuecharacterizations.

The IB parameters calculated within two or more bands of a largerspectral band, by a system embodying the aforementioned ultrasound echoparameterization scheme, are used as input to a tissue characterizationapplication that utilizes parametric data provided by potentially manysources to render a tissue type corresponding to an ultrasound echosegment. An example of such system is described, by way of example, inNair et al., U.S. application Ser. No. 11/689,327.

The tissue characterization output of the above-described system isthereafter used, by way of example, to provide input to a system forclassifying vascular plaque lesions wherein a classification criterionis applied by a plaque classification application to at least onegraphical image of a cross-sectional slice of a vessel to render anoverall plaque classification for the slice or set of slices, covering a3D volume. Such system is described, by way of example, in Margolis etal., U.S. application Ser. No. 11/689,963.

It is noted that while the illustrative embodiment discloses the use oftwo frequency bands, in alternative embodiments more bands are used.Furthermore, the multiple bands may overlap or have band gaps betweenthem.

Systems and their associated components have been described herein abovewith reference to exemplary embodiments of the invention including theirstructures and techniques. It is noted that the present invention isimplemented in computer hardware, firmware, and software in the form ofcomputer-readable media including computer-executable instructions forcarrying out the described functionality. In view of the many possibleembodiments to which the principles of this invention may be applied, itshould be recognized that the embodiments described herein with respectto the drawing figures are meant to be illustrative only and should notbe taken as limiting the scope of invention. Therefore, the invention asdescribed herein contemplates all such embodiments as may come withinthe scope of the following claims and equivalents thereof.

What is claimed is:
 1. A system for characterizing tissue, comprising: aprocessing system in communication with an intravascular ultrasound(IVUS) device, the processing system configured to: compute, fromRF-domain ultrasound echo signal segment data derived from analogultrasound echo signals associated with a region of interest in aportion of a body, spectrally resolved information over an operatingfrequency range resulting from a single transmitted ultrasound frequencyincluding: a power spectrum by performing spectral analysis, and asystem transfer function; determine, in each of at least a firstsub-band and a second sub-band within the operating frequency range, anintegrated backscatter parameter from the power spectrum and systemtransfer function; compare integrated backscatter values from at leastthe first sub-band and the second sub-band to obtain at least oneintegrated backscatter comparison parameter value; apply the at leastone integrated backscatter comparison parameter value to at least onetissue characterization criterion to determine a plaque tissuecharacterization for the tissue in the region of interest; and output animage of the region of interest to a display in accordance with theplaque tissue characterization of the tissue in the region of interest.2. The system of claim 1, wherein the processing system is furtherconfigured to apply an integrated backscatter parameter value from thefirst sub-band directly to the at least one tissue characterizationcriterion.
 3. The system of claim 2, wherein the processing system isfurther configured to apply an integrated backscatter parameter valuefrom the second sub-band directly to the at least one tissuecharacterization criterion.
 4. The system of claim 1, wherein the atleast one integrated backscatter comparison parameter value is basedupon a difference between integrated backscatter parameter values forthe first sub-band and the second sub-band.
 5. The system of claim 4,wherein the difference is a signed value representing the relativemagnitudes of the integrated backscatter parameter values for the firstsub-band and the second sub-band.
 6. The system of claim 1, wherein theat least one integrated backscatter comparison parameter value is basedupon a ratio of integrated backscatter parameter values for the firstsub-band and the second sub-band.
 7. The system of claim 1, wherein theprocessing system determines the integrated backscatter parameters ofthe first sub-band and the second sub-band by computing an apparentbackscatter transfer function (ABSTF) over at least the first sub-bandand the second sub-band.
 8. The system of claim 1, wherein theprocessing system computes the system transfer function utilizing blinddeconvolution.
 9. The system of claim 1, wherein the plaque tissuecharacterization is at least one of fibro-fatty plaque tissue andnecrotic core plaque tissue.
 10. The system of claim 1, furthercomprising the IVUS device configured to obtain the analog intravascularultrasound echo signals and transmit the analog intravascular ultrasoundecho signals to the input.
 11. The system of claim 10, wherein the IVUSdevice is a catheter.
 12. The system of claim 11, wherein an at leastone ultrasound transducer is mounted to a distal portion of thecatheter.
 13. The system of claim 12, wherein a plurality of ultrasoundtransducers are mounted to the distal portion of the catheter.
 14. Thesystem of claim 12, wherein a single ultrasound transducer is mounted tothe distal portion of the catheter.
 15. The system of claim 1, furthercomprising the display.
 16. A system for characterizing tissue,comprising: a processing system in communication with an intravascularultrasound (IVUS) device, the processing system configured to: receiveintravascular ultrasound signals associated with tissue in a region ofinterest of a vessel; compute a power spectrum and a system transferfunction over an operating frequency range resulting from a singletransmitted ultrasound frequency of the IVUS device; determine, in eachof at least a first sub-band and a second sub-band within the operatingfrequency range, an integrated backscatter parameter from the powerspectrum and system transfer function; compare integrated backscattervalues from at least the first sub-band and the second sub-band toobtain at least one integrated backscatter comparison parameter value;and apply the at least one integrated backscatter comparison parametervalue to at least one tissue characterization criterion to determine atissue characterization for the tissue in the region of interest of thevessel.
 17. The system of claim 16, wherein the processing system isfurther configured to output an image of the region of interest to adisplay in accordance with the tissue characterization for the tissue inthe region of interest.
 18. The system of claim 17, wherein the at leastone integrated backscatter comparison parameter value is based upon atleast one of: a difference between integrated backscatter parametervalues for the first sub-band and the second sub-band; and a ratio ofintegrated backscatter parameter values for the first sub-band and thesecond sub-band.
 19. The system of claim 17, wherein the processingsystem computes the system transfer function utilizing blinddeconvolution.
 20. The system of claim 17, wherein the processing systemis further configured to perform at least one of the following: apply anintegrated backscatter parameter value from the first sub-band directlyto the at least one tissue characterization criterion; and apply anintegrated backscatter parameter value from the second sub-band directlyto the at least one tissue characterization criterion.